Galactosyltransferases are the family of enzymes which transfer galactose from UDP-gal to the non-reducing residues of oligosaccharides of various glycoconjugates as well as monosaccharides. The most common sequence, Gal-beta-1 leads to 4G1cNAc, which occurs in glycolipids and as well as in the O- and N-linked glycoprotein oligosaccharides, is formed by N-acetylglucosaminide beta-1 leads to 4 galactosyltransferase. This enzyme has been purified from bovine and human milk, where it interacts with alpha-lactalbumin to form lactose synthase. Alpha-lactalbumin modifies the activity of this galactosyltransferase in such a way that inhibits the transfer of galactose from UDP-galactose to N-acetylglucosamine, either free or linked as a terminal sugar of glycoprotein but facilitates the transfer to glucose or myoinosital. To understand the modulation of the galactosyltransferase activity essential for generating specific cell-surface antigenic determinants, we have undertaken the isolation and characterization of galactosyltransferase cDNA clone. Commercially available bovine galactosyltransferase was repurified, and after acetylation of lysine residues subjected to tryptic digestion. Some of the tryptic peptides were sequenced. A mixed 21-mer and 27-mer nucleotide probes corresponding to two different peptides were synthesized and used as hybridization probes. A bovine cDNA library was constructed in a Okayama-Berg expression vector from the lactating bovine mammary gland poly (A)+RNA. This cDNA library was screened with the labeled oligomer probes. Clones hybridizing to both the probes were further analysed. Authenticity of gal. transf. clone was confirmed by a) translation of the hybrid selected mRNA in frog oocytes and measuring gal. transf. activity and b) transfecting Cos cells with the CDNA clone and measuring transferase activity.